Scaling enemy difficulty

[BryceMehring]

This could be used for dynamic enemy generation and what goes in each wave so that we don't have to hard code what goes in each wave.

[prs9tb]

Just committed rough draft of functionality. Causes some rare, strange bugs in game.lua. (not crashing though)

[BryceMehring]

This was added via 0680eaf.

[BryceMehring]

While it does not crash, it does generate the following error:

Param type mismatch at position 2: expected a 'number' but got a 'nil'
stack traceback:
    [C] in function 'setSpan'
    ./source/libraries/flower.lua:534 in function 'callLoopTime'
    ./source/game/game.lua:211 in function 'startSpawnLoop'
    ./source/game/game.lua:188 in function 'func'
    ./source/libraries/flower.lua:519 in function <./source/libraries/flower.lua:519>

[BryceMehring]

I moved 0680eaf to the waves branch until it is stable enough to merge into master.